Coatings that consist essentially of zinc are conventionally used for the effective protection they provide against corrosion, whether in the automobile sector or in construction, for example. However, these coatings cause weldability problems and are currently in competition with coatings that contain zinc and magnesium.
The addition of magnesium significantly increases the resistance of these coatings to perforating corrosion, and can make it possible to reduce the thickness of these coatings and thus improve their suitability for welding, or also to maintain the thickness of the coating and increase the guaranteed protection against corrosion over time.
In addition, the improvement of the resistance to corrosion is such that it is currently possible to reduce or even to eliminate the use of secondary protection measures such as the use of waxes or mastics in the locations that are the most susceptible to corrosion.
However, coils of sheet with surface coatings of this type can sometimes remain in storage facilities for many months before they are shaped by the final user, during which time this surface must not be altered by the appearance of surface corrosion. In particular, no corrosion may appear, whatever the storage environment, even if the sheet metal is exposed to sun and/or to a moist or even saline environment.
Standard galvanized products, i.e., products whose coatings consist essentially of zinc, are also subject to these constraints and are coated with a protective oil, which is generally sufficient.